fix(robot): Robot.set_default_backend replaces rtbtool's monkey-patch; URDFRobot drops a dead guard

src/roboticstoolbox/bin/rtbtool.py

@@ -321,8 +321,7 @@
 
     # set default backend for Robot.plot
     if args.swift:
-        # an attribute that shadows the class method, see findings
-        Robot.default_backend = "swift"  # type: ignore[method-assign,assignment]
+        Robot.set_default_backend("swift")
 
     # set matrix printing mode for spatialmath
     SE3._ansimatrix = args.ansi  # type: ignore[name-defined]  # from the star import

src/roboticstoolbox/models/URDF/URDFRobot.py

@@ -363,11 +363,9 @@ def _links_from_urdf(urdf: URDF) -> Any:
         elinks.append(elink)
         elinkdict[link.name] = elink
 
-        try:
-            # geometry has no setter, so this raises AttributeError and is ignored
-            elink.geometry = [v.geometry.ob for v in link.visuals]
-        except AttributeError:
-            pass
+        # a visual without a shape, or with a mesh that cannot be found, is
+        # rejected when the URDF is parsed, so every visual here has an `ob`
+        elink.geometry = [v.geometry.ob for v in link.visuals]
 
         if link.collisions:
             shapes = []

src/roboticstoolbox/robot/BaseRobot.py

@@ -77,6 +77,10 @@ class BaseRobot(SceneNode, DynamicsMixin, RobotPlottingMPLMixin, ABC, Generic[Li
     # set when the links are processed in the constructor
     _base_link: LinkType
 
+    #: default graphical backend for every robot of this class, used when the
+    #: robot has not been given its own, see :meth:`set_default_backend`
+    _class_default_backend: Any = None
+
     def __init__(
         self,
         links: list[LinkType],
@@ -823,19 +827,39 @@ def default_backend(self) -> Any:
         Get default graphical backend
 
         - ``robot.default_backend`` Get the default graphical backend, used when
-            no explicit backend is passed to ``Robot.plot``.
+            no explicit backend is passed to ``Robot.plot``.  This is the value set
+            for the robot, otherwise the value set for all robots by
+            :meth:`set_default_backend`, otherwise ``None``.
         - ``robot.default_backend = ...`` Set the default graphical backend, used when
             no explicit backend is passed to ``Robot.plot``.  The default set here will
             be overridden if the particular ``Robot`` subclass cannot support it.
 
         :returns: backend name
         """
-        return self._default_backend
+        if self._default_backend is not None:
+            return self._default_backend
+        return self._class_default_backend
 
     @default_backend.setter
     def default_backend(self, be: Any) -> None:
         self._default_backend = be
 
+    @classmethod
+    def set_default_backend(cls, be: Any) -> None:
+        """
+        Set the default graphical backend for all robots
+
+        :param be: backend name, for example ``"swift"``, or ``None`` to clear
+
+        ``Robot.set_default_backend("swift")`` sets the backend used by
+        :meth:`plot` for every robot, existing or created later, that has not
+        been given one of its own by ``robot.default_backend = ...``.  The robot's
+        own value takes precedence.
+
+        :seealso: :attr:`default_backend`
+        """
+        cls._class_default_backend = be
+
     @property
     def gravity(self) -> NDArray:
         """

tests/test_default_backend.py

@@ -0,0 +1,45 @@
+"""Robot.default_backend, per robot, and the class-level default used by rtbtool"""
+
+import inspect
+
+import pytest
+
+import roboticstoolbox as rtb
+from roboticstoolbox import Robot
+
+
+@pytest.fixture
+def restore_class_default():
+    yield
+    Robot.set_default_backend(None)
+
+
+def test_class_default_reaches_every_robot(restore_class_default) -> None:
+    before = rtb.models.DH.Puma560()
+    assert before.default_backend is None
+
+    Robot.set_default_backend("swift")
+
+    after = rtb.models.DH.Panda()
+    assert before.default_backend == "swift"
+    assert after.default_backend == "swift"
+
+
+def test_instance_value_wins_over_the_class_default(restore_class_default) -> None:
+    Robot.set_default_backend("swift")
+    robot = rtb.models.DH.Puma560()
+    robot.default_backend = "pyplot"
+    assert robot.default_backend == "pyplot"
+    assert rtb.models.DH.Panda().default_backend == "swift"
+
+
+def test_default_backend_remains_a_property(restore_class_default) -> None:
+    # rtbtool used to replace it with a string on the class
+    Robot.set_default_backend("swift")
+    assert isinstance(inspect.getattr_static(Robot, "default_backend"), property)
+
+
+def test_class_default_can_be_cleared(restore_class_default) -> None:
+    Robot.set_default_backend("swift")
+    Robot.set_default_backend(None)
+    assert rtb.models.DH.Puma560().default_backend is None

tests/test_urdf_visual_geometry.py

@@ -0,0 +1,43 @@
+"""Visual geometry of a URDF robot reaches the links, or fails at parse time"""
+
+import pytest
+
+from roboticstoolbox.models.URDF.URDFRobot import _links_from_urdf
+from roboticstoolbox.tools.urdf import URDF, URDFError
+
+_LINK = """<robot name="r"><link name="base">
+  <inertial><mass value="1"/>
+    <inertia ixx="1" ixy="0" ixz="0" iyy="1" iyz="0" izz="1"/></inertial>
+  {visuals}
+  <collision><geometry><box size="1 1 1"/></geometry></collision>
+</link></robot>"""
+
+
+def test_visual_shapes_are_set_on_the_link() -> None:
+    urdf = URDF.loadstr(
+        _LINK.format(
+            visuals=(
+                '<visual><geometry><box size="1 1 1"/></geometry></visual>'
+                '<visual><geometry><sphere radius="0.1"/></geometry></visual>'
+            )
+        ),
+        None,
+    )
+    links, _ = _links_from_urdf(urdf)
+    assert len(links[0].geometry.scene_children) == 2
+
+
+@pytest.mark.parametrize(
+    "visual",
+    [
+        "<visual><geometry/></visual>",
+        "<visual/>",
+        "<visual><geometry><pyramid/></geometry></visual>",
+        '<visual><geometry><mesh filename="package://nope/missing.stl"/>'
+        "</geometry></visual>",
+    ],
+)
+def test_a_visual_without_a_shape_is_rejected_when_parsed(visual: str) -> None:
+    # so that _links_from_urdf never meets a visual without an `ob`
+    with pytest.raises(URDFError):
+        URDF.loadstr(_LINK.format(visuals=visual), None)